The one-electron reduction kinetics of the europium cryptates Eu(2.2.1)3+ and Eu(2.2.2)3+ by the aquo ions [formula-omitted], and [formula-omitted] and the oxidation kinetics of Eu(2.2.1)2+ by [formula-omitted] have been studied by using a polarographic technique in order to examine the effects of encapsulating europium within cryptate cavities upon the reactivity of the Eu(III/II) couple. At 25 °C and an ionic strength m = 0.1, the second-order rate constants (M-1 s-1) for acid-independent pathways are as follows: [formula-omitted]. By comparison of these kinetic data with those for similar reactions involving the [formula-omitted] couple, the rate constant for Eu(III/II) self-exchange, is estimated to increase by factors of ca. 1 X 107 and 2 X 104 upon encapsulation of europium in (2.2.1) and (2.2.2) cryptate cavities, respectively. Estimates of ka equal to ca. 10, 4 X 10-2, and 5 X 10-6 M-1 s-1 (μ = 0.1) for Eu(2.2.1)3+/2+, Eu(2.2.2)3+/2+, and [formula-omitted], respectively, are obtained from the Marcus cross relation. The increases in ka resulting from cryptate encapsulation suggest that nonadiabaticity is not primarily responsible for the extremely low reactivity of [formula-omitted]. The values of kex are shown to be roughly consistent with the Franck-Condon barriers estimated from structural data.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry